it might be what you eat/drink, maybe baby reacted to the breastmilk so you might need to avoid eating dairy products. but to be sure and have a peace of mind, u can bring to doc. they might do stool analysis or just ask you to monitor
You should bring to doctor to check. This is also to avoid recurrence of blood stool and nappy rash. In the meantime, you may want to watch what youve been eating, we may not know if that affect the little one too.